1. Field of the Invention
The present invention relates to a heat sink with fins used for cooling electronic devices, and in particular relates to the heat sink with fins in which heat dissipating fins are jointed on a metal base plate.
2. Description of the Related Art
In order to dissipate the heat generated by a semiconductor chip, which is increasing year by year, there is known a method in which a heat sink is attached in close contact to the semiconductor chip so as to transfer the heat from the semiconductor chip to the heat sink, and dissipate same. A conventional heat sink has a metal flat base plate so as for the semiconductor chip to be closely contacted thereon, and has construction that plural heat dissipating fins of metal thin plates are jointed on the surface opposite to the surface of the base plate on which the semiconductor chip is attached.
In the heat sink with fins having the above-mentioned construction, the heat generated by the semiconductor chip transfers from the semiconductor chip to the metal base plate which is closely contacted thereto, and spreads over the base plate, and thus spread heat is dissipated into air or the designated place by the metal heat dissipating fins which are fixed on the surface of the metal base plate. In addition, there is provided a one-piece heat sink made of forged aluminum as one of the conventional heat sinks. However, there is a problem in which the one-piece heat sink made of forged aluminum do not attain enough heat dissipating effect, because required number of heat dissipating fins are not formed due to the technical difficulty by forging to shorten the pitch between the heat dissipating fins for the required level of fin-density.
In order to solve the above-mentioned problem and obtain much more excellent heat dissipating effect, there is proposed a method instead of a one-piece heat sink, in which a base plate and heat dissipating fins are separately manufactured and the heat dissipating fins are jointed on a surface of the base plate by means of brazing or mechanical joint.
The mechanical joint in which the heat dissipating fins are jointed mechanically on a surface portion of the base plate is superior to the brazing, because manufacturing cost of brazing is higher than that of mechanical joint.
As shown in FIG. 4, when the size of the semiconductor chip 30 and the base plate 22 are compared, the semiconductor chip 30 is much smaller than the base plate 22. Therefore it is necessary to spread the heat generated by the small semiconductor chip 30 all over the large base plate 22 by using the base plate made of materials having high heat conductivity such as copper, aluminum or the like. In particular, since the copper is excellent in heat conductivity, the copper is known as a material for the base plate of the heat sink with fins. However, since the copper is heavy and the copper base plate must be in direct contact with the semiconductor chip, there is required some reinforcing construction (for example: applying a jointing device for protecting a circuit board or the like) to reduce the damage that the weight of the copper base plate imposes onto the semiconductor chip.
Furthermore, electronic devices become remarkably lighter and smaller, which makes it difficult to use the copper base plate because the chance to use such heavy copper base plate is reducing and it becomes difficult to secure the space for installing the copper base plate. On the other hand, when the semiconductor chip becomes more integrated and the processing capacity becomes higher, the heat generated by the semiconductor chip becomes larger. Therefore, further higher heat dissipating effect is required for stable operation of the semiconductor chip or the like.
As described above, it becomes difficult to use the copper base plate due to the heavy weight thereof in spite of the excellent heat conductivity. Therefore, when aluminum is to be used for material of the based plate, it is necessary to satisfy the requirement of further increasing heat dissipating effect of the heat sink.
In the heat sink having construction that the heat dissipating fins are mechanically fixed on a surface of the base plate, the base plate has the following temperature distribution when the heat dissipation is in the stable condition. More specifically, the temperature is the highest in the central portion of the base plate to which the semiconductor chip is attached in close contact and, the temperature becomes lower as the portion in the base plate moves farther from the semiconductor chip.
In addition, the heat dissipating fins which are fixed and thermally connected to a surface of the base plate has the following temperature distribution. More specifically, the temperature is the highest in the heat dissipating fins locating in the central portion of the base plate to which the semiconductor chip is attached in close contact, and the temperature is the lowest in the heat dissipating fins locating in the end portions of the base plate which are far apart from the central portion.
Furthermore, the heat dissipating fin itself has the following temperature distribution along the height direction. More specifically, the temperature is the highest in the bottom portion of the fin which is fixed and thermally connected to the base plate and, the temperature becomes lower as the portion moves farther from the bottom portion.
It proves that the heat is transferred by heat conduction in the heat dissipating fin.
However, it is desirable to minimize difference of temperature across the entire portion of the heat sink with fins, from the heat dissipating efficiency point of view. As described above, lately, the heat generated by the semiconductor chip is increasing, and at the same time, the semiconductor chip is more downsized, which tends to rapidly increase the heat density. Therefore it is urgently required to improve heat dissipating efficiency of the heat sink with fins (for example, increasing density of the heat dissipating fins fixed on the base plate, or using a heat pipe in the base plate, or the like). In particular, it is strongly demanded to increase heat dissipating efficiency by reducing difference of temperature in the portions of the above-mentioned heat sink with fins.
As mentioned above, there is a problem in which a one-piece heat sink made of forged aluminum does not attain enough that dissipating effect, because the required number of the heat dissipating fins are not formed due to the technical difficulty by forging to shorten the pitch between fins for the required level of fin-density. Furthermore, there is also a problem in which using the heavy copper base plate becomes difficult due to the heavy weight thereof even if the heat conductivity is excellent. Therefore, it is strongly demanded to increase heat dissipating efficiency of the heat sink with fins having construction that the heat dissipating fins are mechanically fixed on a surface of the aluminum base plate.
However the heat sink having construction that the heat dissipating fins are mechanically fixed on a surface of the aluminum base plate has problem in heat dissipating efficiency, because there are temperature difference both in the base plate and the heat dissipating fins.
There is provided according to the invention a heat sink with fins comprising groups of heat dissipating fins which comprise at least two groups of heat dissipating fins, which are made of at least two kinds of metals having different heat conductivity, said heat dissipating fins of each group being made of same metal, respectively and a metal base plate on which surface portion said groups of heat dissipating fins are densely jointed, wherein a part of said at least two groups of heat dissipating fins are integrally formed with said metal base plate.